Mold design for ceramic casting



Nov. 29, 1966 J. DEAN 3,288,424

MOLD DESIGN FOR CERAMIC CASTING Filed Nov. 1, 1962 INII'ENFOR.

Jb/m Dean United States Patent ware Filed Nov. 1, 1962, Ser. No. 234,785 1 Claim. Cl. 249-108) This invention relates generally to the manufacture of ceramic objects and more particularly to a casting mold and method for forming such objects.

Large electrical insulating porcelain bushings are commonly formed by slip casting, in which process, casting material in the form of a thick, creamy slip is poured into the object-forming cavity of an upright, plasterof-Paris mold and allowed to stand for a period of time sufficient for the slip to set or harden. The porous mold walls absorb water from the slip during this setting period, requiring a continuous addition of liquid slip, preferably under pressure, in order to prevent shrinkage of the ceramic shape. The solid particles of the slip in the mold cavity gradually coalesce as moisture is absorbed by the porous mold walls; and when the newly cast shape has sufficiently solidified, it is removed from the mold, to be thereafter dried. and fired according to well-established procedures familiar to the art.

Molds heretofore used in forming large porcelain bushings by the above type process have a casting cavity of the same size and shape as the bushing being formed. Slip is fed into the cavity through one or more casting wells which extend through the wall of the mold to the usable surface of the object itself. Since the opening in a casting well is small in relation to the over-all size of the mold cavity, additional slip which is fed through the casting well during the setting period, moves at a different rate than the remainder of the slip in the cavity. This causes the slip near the casting well to set in a difierent manner than the rest of the slip, with the result that the structure of the clay in this area differs from that of the rest of the object. Since this area of non-uniformity extends into the object, it is not eliminated when the excess material from the casting well is trimmed from the surface of the object following removal from the mold.

The non-uniformity of structure so caused adversely affects the mechanical and electrical strength of the porcelain bushings. Usually, the defect appears as a high shrinkage area which may not become apparent until after the ware has been fired, when it shows up in the form of cracks, depressions, warpage, or a combination of these defects. This non-uniformity constitutes a major cause of cracking and warpage in long cylindrical pieces. In addition, open voids sometimes appear beneath the surface of the object in the area of the casting well and such defects cannot readily be detected without the use of destructive inspection which destroys the ware.

In the mold and method of the present invention, an over-all mold cavity is provided. having a ware-forming portion and an auxiliary cavity or extension of the wareforming cavity through which slip is supplied from a reservoir. This slip is admitted to the auxiliary section of the mold cavity through a casting well at a location remote from the ware-forming portion of the cavity. When the casting process is complete, an object has been formed conforming to the mold cavity, that is, a single object defining the shape of the ware, plus the shape of the auxiliary cavity portion, with the configuration of the latter being of substantially the same cross-sectional area as that of the ware-forming portion of the cavity at the location of the juncture between the auxiliary and wareforming portions. To complete the form of the ware, it is necessary to trim away that portion of the molded ice object which was formed by the auxiliary cavity portion and by so doing, the structurally non-uniform portions of the object are discarded with the auxiliary portion giving a piece of ware having the desired structural characteristics.

Another object of the invention is to provide a casting mold for forming large insulating bushings and other similar shapes which are structurally superior to those of the prior art.

These and other objects and advantages of the invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a sectional elevation view showing the casting mold of this invention;

FIGURE 2 is a partial elevation view, also partly in section, showing the top portion of a newly cast shape in the mold of FIGURE 1 after the top portion of the mold has been removed, and before the shape has been trimmed; and

FIGURE 3 is an elevation view, partially in section, showing a completed electrical insulating bushing formed in the mold of FIGURE 1.

Referring to FIGURE 1 of the drawings, an upright porous mold 10 is illustrated having an elongated upright casting cavity partially defined by the surface 12 therein. The mold 10 has two mating outer mold sections 14, a lower base ring 16, a central core section 18, and an upper cap or auxiliary cavity forming member 20, which, when assembled as shown, define the concentric casting cavity partially defined by surface 12. These various mold sections are all formed of porous mold material such as plaster-of-Paris. The interior surface 21 of the upper cap 20 defines a cavity which has a lower opening in communication with the upper opening of the mold cavity 12 and of substantially similar cross-sectional area at the junction of the two cavity forming portions.

The mating outer mold sections 14 of the mold 10 are held together in radially spaced relation to the central core section 18 by the bands 22. A reservoir 25 is supported vertically above the cap member 20, in fluid. delivering communication with the cavity within the cap 20 through the casting well 24 to provide an extra supply of slip by gravity feed and effect a greater hydrostatic pressure upon the slip confined within the cavity defined by surface 12.

In forming a ceramic object in accordance with this invention, the casting slip is fed into the mold cavity from the reservoir 25 through the casting well 24 until the mold cavity is completely filled and an additional quantity substantially fills the reservoir 25. The casting slip contained in the reservoir 25 serves both to provide a continuous supply to the mold cavity to replace the shrinkage of water absorbed by the porous plaster mold walls and a hydrostatic pressure. It will be observed that the casting well 24, through which the slip is admitted to the mold cavity, enters the auxiliary cavity portion defined by the surface 21 at the location most remote from the ware-forming lower cavity portion.

An alternative to the gravity fed slip supply shown would be the connection of a supply conduit to the upper opening of the reservoir 20 that would provide a continuous supply of slip to the mold. under a predetermined pressure.

When the mold ceases to absorb shrinkage water, no further supply of slip is required since no further size reduction will occur as a result of further drying of the ware. As the solid particles of the slip acquire sufficient cohesion to constitute a soft semi-solid, commonly referred to as greenware, the latter may be removed from the mold for additional processing and drying preparatory to firing. While the object remained in the semisolid greenware state, the portion thereof that was formed by the cavity within the cap portion is trimmed. from the ware and discarded, to bring the ware to the completed molded shape as shown in FIGURE 3. This may be accomplished by removing the ware from the mold and trimming away the top portion along the line AA, but this operation can be more simply expedited by removing the auxiliary cavity forming portion 20, and the core 18 as shown in FIGURE 2, and trimming across the top of the mold halves 14 with the upper surfaces 26 thereof serving as a guide for the cutting edge.

In this casting method, the slip is fed to the cavity from the reservoir 25, through a casting well which is located remotely from the ware-forming portion of the cavity and accordingly the latent structural and electrical defects of the cast object structure adjoining the casting well occur in that portion formed by the auxiliary cavity portion defined by the surface 21 within the cap 20. It is this portion of the cast object containing the possible defects which is discarded to furnish a completely formed greenware object free from this principal cause of defective finished ware.

While a particular embodiment of the invention has been shown and described, it is not intended that the invention be limited thereto, but should include various 25 modifications within the scope of the appended claim.

What is claimed is: A mold for casting an elongated cylindrically shaped porcelain electrical bushing comprising a lower elongated vertically aligned ware-forming portion open at the upper end thereof and defining a ware-forming cavity of the same size and shape as said bushing,a removable auxiliary section presenting a wall surface defining an auxiliary cavity extending upwardly from said ware-forming portion upper end, the horizontal cross section of said auxiliary cavity immediately above said ware-forming portion being of substantially the same area as the cross section of the ware-forming cavity immediately below said auxiliary section, whereby an abrupt change in horizontal cross section is avoided at the juncture of said wareforming cavity and said auxiliary cavity; a casting well extending through said auxiliary section and. communicating with the uppermost portion of said auxiliary cavity.

References Cited by the Examiner UNITED STATES PATENTS 1,304,571 5/1919 Kirk.

1,351,599 8/1920 Wolff 26486 2,261,583 11/1941 Hemphill.

2,288,661 7/1942 -Wadman 28486 3,225,414 12/1965 Dean.

ROBERT F. WHITE, Primary Examiner. ALEXANDER H. BRODMERKEL, Examiner. R. B. MOFFITT, Assistant Examiner; 

